Non-ionizing radiation is radiation in that part of the electromagnetic spectrum that lacks sufficient energy to cause the ionization of an atom or molecule.
The electromagnetic spectrum is the range of all possible electromagnetic radiation. Electromagnetic (EM) radiation is a form of energy consisting of oscillating electrical and magnetic waves that travel in conjunction with each other. EM radiation takes a variety of forms depending on the wave nature with which they travel. This is described by two variables, wavelength and frequency. The wavelength (λ) is the distance between adjacent peaks or troughs of the wave. The frequency (f) of a wave is the number of wavefronts that pass some given point in a designated period of time. Any unit of linear measure can be used to express the wavelength of a wave: for example, meter (m), centimeter (cm), nanometer (nm), or Ångström (Å). The choice of units depends to some extent on the type of wave being discussed. Radio waves, for example, typically have wavelengths ranging from a few millimeters to more than 100 kilometers, whereas gamma rays have wavelengths of less than 10 picometers (pm; a trillionth of a meter). The frequency of a wave is most commonly expressed in hertz (Hz), defined as one cycle per second. According to that definition, any given point on a wave with a frequency of 1 Hz passes some point in space once every second. As with wavelengths, the frequencies of waves varies significantly across the electromagnetic spectrum from a minimum of less than 10 Hz for extremely low frequency (ELF) radio waves to a maximum of more than 1024 (1 followed by 24 zeroes) for gamma radiation.
The energy of a wave is directly proportional to its frequency and inversely proportional to its wavelength. Thus, gamma radiation and X rays, both of which have very high frequencies, have high energies, whereas radio waves and microwaves, with relatively low frequencies, have relatively low energies. The particle that carries energy in an electromagnetic wave is the photon, whose symbol is the Greek letter gamma, γ. The energy of a photon is given by the λ = hf, where h is a constant known as Planck's constant. This expression illustrates the point that the greater the frequency of a wave, the greater its energy.
The atoms and molecules that make up matter are always in motion. That motion can take any one of three forms: translational, rotational, or vibrational. Translational motion involves the movement of a particle from one place to another, whereas rotational motion involves the spinning of a particle around a central axis. Vibrational motion is the back-and-forth motion of the components of a particle similar to the stretching and relaxation of a spring. The electrons present in atoms are also in motion; they spin around the nucleus, travel back and forth between atomic nuclei, or move to higher or lower energy levels in an atom. When radiation passes through matter, it has a tendency to increase the energy of particles that make up the matter. If incoming radiation carries a large amount of energy, as is the case with gamma radiation or x rays, it may be sufficient to expel one or more electrons from its normal atomic orbit, creating an ion. An ion is a particle that has gained or lost an electron. Radiation with sufficient energy to ionize atoms is called ionizing radiation. Other types of radiation may carry too little energy to actually expel electrons from an atom. They may, however, produce other effects in matter, such as increasing the translational, rotational, or vibrational energy of particles. Radiation with insufficient levels of energy to produce ionization is called non-ionizing radiation.
The boundary between ionizing and non-ionizing radiation in the electromagnetic spectrum is not clear. Generally speaking, radiation with frequencies higher than about 1015 Hz are regarded as ionizing, whereas radiation frequencies below that point are regarded as non-ionizing. The dividing point between the two types of radiation, then, is somewhere within the ultraviolet range of the electromagnetic spectrum.
The form of non-ionizing radiation of most frequent health concern is so-called near ultraviolet (UV) radiation. That term refers to UV radiation with energies too low to produce ionization in the body, but sufficiently high to produce other health results. In particular, near UV radiation has sufficient energy to produce burn-like effects on the skin and in the eyes. The sunburn that is typically associated with overexposure to sunlight is one such health effect, whereas inflammation of the eye is another. Sunburn is eventually responsible for the development of skin cancers which can be fatal, whereas inflammation of the eye may eventually evolve to become cataracts. The World Health Organization (WHO) estimates that exposure to near UV radiation may be responsible for between two and three million cases of basal cell and squamous cell carcinomas annually and 130,000 cases of malignant melanoma. Overall, WHO estimates that 66,000 people die each year worldwide from one or another form of skin cancer. It also suggests that up to 20 percent of the 12 to 15 million cases of cataracts around the world each year can be attributed to near UV radiation exposure.
Radiation with somewhat less energy than that of near UV radiation includes light and infrared (IR) radiation, sometimes grouped together as optical radiation. Health effects similar to those found with near UV radiation are also found for optical radiation, but to a significantly lesser degree. Such effects include skin reactions, mild skin burns, and damage to the cornea. These health effects are likely to be found only among individuals who are exposed to unusually high intensity levels of optical radiation for extended periods of time.
The last category of radiation, which includes microwave and radio radiation, is sometimes called EM field radiation. The health effects resulting from exposure to EM field radiation consists primarily of modest heating effects on the body, as well as possible effects from exposure to electrical and magnetic fields. These effects may include disruption of nervous and muscular responses and feelings of disorientation and nausea.
Probably the most contentious issue about the health effects of exposure to non-ionizing radiation in the early 2000s has to do with cell phone use. Cell phones use a technology that involves radiation in the EM field radiation range and given the intensity with which some people use cell phones, concern has arisen about the possible health effects of long-term exposure to EM radiation. A large number of studies have been conducted on this question with, thus far, no clear results. Some research has shown a moderate increase in the rate of cancer among cell phone users, whereas other studies have been unable to replicate these findings. No other health effects related to cell phone use has been confirmed. Researchers have some hope that one large-scale study, the Cohort Study of Mobile Phone Use and Health (COSMOS), will answer a number of fundamental questions about the relationship of cell phone use and health effects. The study was initiated in March 2010 and is expected to continue for at least 20 years. For the present, a number of national and international organizations have said that the forms of EM field radiation used in cell phone technology should be considered a possible carcinogen, suggesting that evidence is not yet convincing for the role of cell phones in cancer production but that due caution should be relevant to the use of these devices.
See also Melanoma ; Radiation ; Radiation exposure ; Skin cancer ; World Health Organization .
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David E. Newton, EdD